Volume control



Sept. 27, 1932. H. A. WHEELER 1,879,862

VOLUME CONTROL I Original Filed July 7. 1927 I- I I l I L l I I m V-Ph Q I n i a l 5 2} x u\ 1 i I In INVENTOR Harold A. Meals)",

ATTORNEYS Patented Sept. 27, 1932 UNITED STATES PATENT OFFICE HAROLD A. WHEELER, OF JACKSON HEIGHTS, NEW YORK, ASSIGNOB TO HAZELTINE CORPORATION, A CORPORATION OF DELAWARE VOLUME CONTROL Original application filed July 7, 1927, Serial No. 203,879, and in Great Britain July 3, 1928. Divided and this application filed-November 10, 1930. Serial No. 494,559.

This invention relates to amplifiers and more particularly to amplifiers utilized in modulated carrier-current signaling systems wherein the limit of amplification is automatically maintained substantially at a predetermined level.

This application is a division of application Serial No. 203,879, filed July 7 1927 The principal object of this invention is to obtain a substantially uniform response from an amplifier in spite of variations in signal intensity.

When amplifiers are employed for amplifying a signal voltage, it becomes desirable for various reasons, to control automatically the amplitude of this amplified signal voltage. To this end the present invention provides means for effecting automatic amplification control. Such an arrangement, for example, is particularly advantageous in radio receivers such as are employed for receiving broadcasting signals, because it prevents the overloading of the last amplifier stage of the receiver, which overloading would result in distortion of the reproduced signal, as well as in loud and harsh reproduction.

Another advantage of the invention resides in uniform reproduction of the amplified signal irrespective of whether the carrier-current signal is received from a nearby station or from a distant station, or from'a high power or from a low power station, since it has been found in the case of former radio receivers that when the receiver was repro ducing strong signals as from a nearby or high power station, the audibly reproduced signal was very loud, whereas when the signal was received from a distant or low power station, it was relatively weak, with the result that if signals were to be reproduced uniformly from both near and distant stations, and from high power and low power stations, it became necessary to readjust some volume control means in the receiver to compensate for these unequal signals.

It has been a common experience in the use of former radio receivers, that the reproduced signal was not uniform, due to the phenomenon of fading, whereby the received signal occasionally or periodically became much weaker, or faded almost to the point of inaudibility. Since the present invention provides an amplifier which automatically compensates for inequalities in the received carrier-current, when fading takes place, the degree of amplification is correspondingly increased and the reproduced signal maintained at its former volume so that a listener is unaware that variation of the received car-' rier current signal is occurring. This automatic compensation for signal fading is especially advantageous in commercial radiotelephone andlike systems.

A further advantage is the saving in plate current which is automatically efiected during the reception of the powerful signals, for the reason that this invention incidentally provides means for reducing the plate current of one or more amplifying tubes as the signal strength increases.

. The above advantages are attained in accordance with the present invention by employing in the signal receiver a rectifying, or detecting, device preceded by one or more stages of carrier frequency amplification and impressing the potential of a point in the detector output circuit upon a control circuit of the carrier frequency amplifier. This is accomplished by connecting a point in the detector output circuit to the control electrode of one or more of the carrier-frequency amplifying tubes. There is included in the detector output circuit the usual audiofreqency transformer for coupling the detector to the audio-frequency amplifier. The volume controlling connection is taken at a. point between the primary winding of the transformer and a resistance element which is included in the circuit between the lower end of this primary winding and the detector cathode.

A feature of the invention is a circuit arrangement and means whereby the potential upon the grid of the volume controlling tube is the fall in potential across the resistance in the detector anode circuit. An increase of the signal strength tends to increase the detected current, thereby increasing the negative bias on the control electrode of the volby a uni-control arrangement.

ume controlling tube; which results in decreased amplification.

The drawing illustrates a radio receiver comprisin two stages of radio-frequency amplification, a detector and two stages of audio-frequency amplification. The two radio-frequency stages comprise two thermionic amplifiers 90 and 91 coupled in tandem, the output of tube 91 being coupled to a three-electrode detector tube 94, and the input of tube 90 being coupled to an antenna 88. The tubes are coupled in a well-known manner by transformers 89, 92 and 93, the secondary windings of which are tuned by variable condensers G G and C The variable condensers are grounded to eliminate undesirable capacity effects and to enable the condensers to be ganged together and operated The primary windings 150 and 151 of transformers 92 and 93 have connected at their low voltage ends, similar windings 152 and 153, the upper ends of which are connected respectively to the grids of tubes 90 and 91 through condensers 154 and 155. The function of these latter condensers is to neutralize the grid-plate capacity of the tubes in a well-known manner, thus preventing the occurrence of sustained oscillations, and otherwise improve the effectiveness of the present invention. The neutralization system is more fully described in S. Patents 1,489,228 and 1,533,858, issued to L. A. Hazeltine.

The three-electrode detector tube 94:, which is of the C-bias type, has in its anode circuit a battery 100, the positive side of which is connected to the detector anode through the primary winding of an audio-frequency transformer 99. The negative side of battery 100 is connected through a resistor 101 to the positive terminal of a B-battery 156 which is a common anode battery for all the tubes. A suitable negative potential is maintained on the grid 96 of the detector tube by means of a C-battery 97, and this grid potential may be adjusted by a potentiometer 98 connected across the detector cathode. The negative terminal of the C-battery is connected to the secondary winding of transformer 93.

The audio-frequency amplifier comprises thermionic amplifiers 105 and 108 coupled in tandem by an audio-frequency transformer 107, the secondary winding of which has shunted across it a resistance 110, the effect of which is to improve the uniformity ""of amplification over the voice frequency range. The cathode of amplifier 105 has in its circuit a rheostat 106 for controlling the cathode current. The output of amplifier 108 is connected to a loudspeaker 109, or if desired, to some other type of receiving device such as a meter or telephone system. The first audiofrequency amplifier 105 is coupled to the out.

put of the detector by having its input connected to the secondary winding of transformer 99.

The cathodes of all the tubes are heated by a common A battery to which the cathodes are all connected in parallel.

F or controllin the amplitude of the radio-frequency vo tage applied to the input circuit of detector 94, a conductor 111 is connected between point 112 common to a terminal of resistor 101 and battery 100, and the secondary winding of transformer 89; thereby applying the potential of point 112 to the grid of the first radio-frequency amplifying tube 90. A by-pass condenser 113 connecting the conductor 111 to the filament system serves to filter out and reject any audio-frequency currents present in the circuit including the conductor 111, thereby insuring that these currents have no effect on the grid of the vacuum tube 90.

Battery 100 is in a sense the source of negative biasing voltage applied to the controlgrid, or control-electrode of the radio-frequency amplifying tube 90. It will be noted that the anode voltage in the detector circuit is furnished by the two batteries 100 and 156, whereas the anode ing tubes is supplied by mattery 156 alone. Due to the magnitude of resistance 101, a suitable value for which will be indicated later, and to the greater total battery volt age applied to the anode of the detector, with respect to that of the other tubes, the current flowing through resistance 101 is sufficient to produce enough potential across the resistance to maintain point 112 negative with respect to the, negative terminal of battery 156. Since point 112 is negative with respect to the cathodes and since battery 100 has a voltage less than shown for battery 100 in the drawing), it follows that the anode potential of the de tector is less than that of the anodes of the remaining tubes.

There is provided in the plate circuit of tube 90 a milliammeter 116 for indicating the anode current of that tube. A switch 115 serves to connect the lead 111 to the cathode of tube 90 when desired. The functions of the milliammeter and of the switch will be more fully disclosed later.

In this embodiment, condensers 104 and 113 may be of 0.0005 microfarad and 1 microfarad capacity, respectively; while resistance 101 may have a value of 0.5 megohm. This invention is not limited to the use of these values, but they are given simply to indicate how good results may be obtained.

In the operation of this receiver, a signal received by the antenna 88 is successively amplified through the two radio-frequency amplifier stages. The audio-frequency signals with which the rado-frequency carrier Wave is modulated, are then detected in the well-known manner by the grid-bias detectpotential of the remainl or 94. The audio-frequency signals are successively repeated by the audio-amplifying stages comprising thermionic amplifiers 105 an .108, after which they are reproduced as sound by sound reproducer 109.

A detected signal current flowing through resistance 101 causes point 112 of the detector anode circuit to become more negative relative to its cathode, and hence causes the potential of the grid of radio-frequency amplifier 90 to become more negative relative to its cathode, thus decreasing the amplification of this amplifier stage. The stronger the incoming signals become, the more negative becomes the grid of amplifier 90, and the more the amplification is reduced. Conversely, when the signals become weaker the amplification increases. The result of this is that the strength of the signals impressed upon the detector remains nearly uniform for widely different signal strengths.

The automatic volume control not only comprises for fluctuation in the strength of the incoming signals, but also compensates for variations or reduction of the A and B voltage supply. A drop in the supply voltage, for example, reduces the current through resistance 101, thereby reducing the negative potential on the grid of tube 119 and increasing the amplification. Conversely,v an increase of the supply voltage makes the grid of the first radio-frequency amplifier more negative.

In this manner, the radio frequency voltage applied to the input of the rectifier detector is maintained at nearly constant predetermined value, and the volume of the reproduced signal is substantially uniform under all conditions; The degree of volume of the redroduced signal is then determined by adjustment of rheostat 106, which controls the heating current in the filament of the first audio-frequency amplifying tube 105.

In adjusting the receiver it is necessary to determine the correct setting of the detector grid potentiometer 98. This adjustment should be made while there is no signal being recived, as follows: First, the switch 115 is closed, and the normal plate current of the tube 90 is noted 011 milliammeter 116. Then the switch is opened, thus placing the control circuit in operation. In general, the plate current of vacuum tube 90 will change when the switch is opened, since the grid voltage of this tube is dependent upon the control circuit. By varying the grid voltage of the detector by potentiometer 98 the plate current of tube 90 is then adjusted to the normal value; and the apparatus is ready for operation. Upon receipt of an amplified signal at the detector, the effect of the control circuit is to decrease the plate current through milliammeter 116, thereby reducing the amis tuned to the signal frequency, a minimum plification of the tube 90. When the receiver in the system illustrated in the drawing, ifthe detector cathode is maintained at the same potential as the cathode of the volume control tube, or tubes.

It is possible to employ in place of the three-electrode detector, a two-electrode rectifier or detector, in the manner set forth in my above mentioned application Serial No, 203,879. An advantage of the three-electrode detector over the two-electrode type is its greater response to weak signals. A particular advantage of the three-electrode detector, as shown combined with an audio frequency transformer, is that the transformer step-up ratio serves to improve the sensitivity of the detector and therefore of the entire receiver.

The volume controlling system may, if desired, be applied to more than one amplifyin stage, in the manner disclosed in my co-pen ing application Serial No. 203,879, filed July I claim:

In a signaling system, a carrier frequency amplifier comprising a repeating element having a cathode and a control electrode, a detector coupled to said amplifier, said detector having a cathode and an anode, a first source of anode potential common to said repeating element and said detector, the series combination of an output transformer, a second source of direct voltage and a resistance in the output circuit of said detector, the primary winding of said transformer being connected to said detector anode, said resistance being connected to the positive terminal of said first source of anode potential, and a connection from a point between said resistance and said second source of potential to said control electrode whereby the amplification of said amplifier is regulated automatically.

' In testimony whereof I aflix my signature.

. HAROLD A. WHEELER.

cnnrmcarn or CORRECTION;

Patent No. 1,879, 862. t September 27, I932.

HAROLD a. mama.

it is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, line 93, for a the word "mattery" read battery; and line 128, for themisspelled compound word "rado-frequency" read radio-frequency; page 3, line 23, for "comprises" read compensates; line 40, for the misspelled word "redroduced" read reproduced; line 48, for "recived" read received; and line 64, strike out the words "is tuned to the signal frequency, a minimum"; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

' Signed and sealed this 29th day of November, A. D. 1932.-

, I, M. J. Moore, (Seal) Acting Commissioner of Patents. 

